1. Field of the Invention
The present invention relates to a method of forming a semiconductor device, and more particularly, to a method of forming a semiconductor device having different work function layers for tuning threshold voltages.
2. Description of the Prior Art
In the current semiconductor industry, polysilicon has been widely used as a gap-filling material for fabricating gate electrodes of metal-oxide-semiconductor (MOS) transistors. However, the conventional polysilicon gate also faces problems such as inferior performance due to boron penetration and unavoidable depletion effect which increases equivalent thickness of gate dielectric layer, reduces gate capacitance, and worsens driving force of the devices. In replacing polysilicon gates, work function metals have been developed to serve as a control electrode working in conjunction with high-K gate dielectric layers.
Typically, threshold voltage in conventional planar metal gate transistors is adjusted by the means of ion implantation. With the trend in the industry being towards scaling down the size of the metal oxide semiconductor transistors (MOS), three-dimensional or non-planar transistor technology, such as fin field effect transistor technology (FinFET) has been developed to replace planar MOS transistors. Nevertheless, threshold voltages in current FinFET cannot be easily adjusted by using ion implantation. Hence, how to resolve this issue in today's FinFET architecture has become an important task in this field.